1. Field of the Invention
The present invention relates to an active matrix substrate for use in displays such as liquid crystal displays and organic electroluminescent displays.
2. Description of the Background Art
Well known electro-optical displays, such as liquid crystal displays and organic electroluminescent displays, are active matrix types which have a plurality of switching elements such as thin-film transistors (TFTs) arranged on an insulated substrate and apply voltage independently to each pixel.
Especially in displays using liquid crystals as electro-optical elements, it is important to achieve active matrix substrates with a large display area of each pixel, i.e., with high aperture ratios, for bright and high-quality display.
One example of such active matrix substrates with high aperture ratios has been suggested in Japanese Patent Application Laid-open No. 9-325330 (1997) (FIGS. 1 and 2). This patent document has disclosed a structure in which an organic interlayer insulating film is formed to cover both gate and source signal lines, and pixel electrodes are formed on the organic interlayer insulating film. This active matrix substrate allows the pixel electrodes to overlap the signal lines, thereby increasing the aperture ratios of liquid crystal displays as well as shielding electric fields caused by the signal lines and thus inhibiting imperfect orientation of liquid crystals.
However, the organic interlayer insulating film in the above active matrix substrate is water absorbent and porous and thus may have an increased moisture density. If, in this condition, voltage is applied to TFTs, electrical charges will be induced on the surface of a channel region of a semiconductor layer under the influence of polarization of the interlayer insulating film resulting from moisture. This deteriorates off-current characteristics of the TFTs and thereby causes display defects such as display unevenness.
One example of methods to solve this problem has been suggested in Japanese Patent Application Laid-open No. 2000-221488 (FIGS. 1, 4, and 5).
An active matrix substrate disclosed in this patent document is such that an inorganic passivation film of, for example, silicon nitride is formed under an organic interlayer insulating film to protect a channel region of a semiconductor layer. Besides, the organic interlayer insulating film is made of an organic material containing water absorbing or moisture adsorbent particles. This prevents deterioration in the off-current characteristics of TFTs caused by polarization resulting from moisture.
However, in the structure disclosed in this patent document, etching of the inorganic passivation film may result in excessive etching to an insulation film located under the passivation film, thereby causing short circuits between pixel electrodes and auxiliary capacitive electrodes.
Thus, in the active matrix substrate disclosed in Japanese Patent Application Laid-open No. 2000-221488, it is necessary to form contact holes by, for example, dry etching the inorganic passivation film, in order to establish connection between the drain electrodes and the pixel electrodes. At this time, if there are pin holes in a metal film used to form source and drain electrodes, especially in areas of the metal film where contact holes are formed, a gate insulating film under the metal film may be etched through the pin holes simultaneously as the process of removing the passivation film by dry etching. This causes the contact holes to reach the underlying auxiliary capacitive electrodes through the gate insulating film and thereby causes short circuits between the pixel electrodes and the auxiliary capacitive electrodes, resulting in the occurrence of display defects.